The invention relates to controlling the rate at which a liquid sample flows through a system.
Many processes involve preparation of a liquid sample, flow of the sample along a loading path to a processing device, and flow of the sample through the device. Particularly important devices of this kind are liquid chromatography (LC) columns, which are widely used to separate, identify, purify and quantify components of mixtures. Other such devices include detectors and reaction chambers. In such processes, reducing the volume of the liquid sample offers important potential benefits. For example, high performance LC (HPLC) typically uses columns 2.0 to 10, e.g. about 4.6, mm in diameter, whereas microcolumn LC (μLC) typically uses columns 2.0 mm or less in diameter and samples having a volume of less than 500 nL. However, as the sample volume decreases, it becomes increasingly difficult to precisely control the volume of the sample and to achieve the square pulse shape of the sample which is desirable. Consequently, systems for preparing and delivering samples to conventional HPLC columns are not satisfactory for use μLC systems. Attempts have been made to develop injection valves and methods for μLC systems. See, for example, Vissers et al, J. of Chrom A, 746, p 1, (1996); Bakalyar et al, J. Chrom. A, 762, p 167, (1997); and Foster et al, J. Chrom. A, 869, p 231, (2000), and the valves commercially available from VICI Valco Instruments, Rheodyne and Upchurch Scientific. Valve designs include both external and internal sample loops. Injection volumes of less than 100 nL are typically achieved using valves with internal sample loops where a groove in the rotor serves as the loop. Larger injection volumes can be achieved with either internal loops or external loops connected to the valve ports.